Semiconductor materials exhibit controllable optical and electrical properties, such as conductivity, over a wide range. Such control is enabled by use of dopants, which are impurities introduced into the crystalline lattice of the semiconductor material to serve as sources of electrons (negative charges) or holes (positive charges). Controllable doping enables the fabrication of a wide range of semiconductor devices, e.g., light-emitting diodes (LEDs), lasers, and transistors.
Nitride-based semiconductors such as gallium nitride (GaN) and aluminum nitride (AlN) are of great interest technologically, in part because of their wide bandgaps. Controllable and repeatable doping of these materials enables the fabrication of light-emitting devices, such as LEDs and lasers, that emit light at short wavelengths, i.e., at blue, violet, and even ultraviolet wavelengths. Moreover, n- and p-type nitrides can be utilized in the fabrication of transistors suited for high power and/or high temperature applications. In an n-type semiconductor, the concentration of electrons is much higher then the concentration of holes; accordingly, electrons are majority carriers and dominate conductivity. In a p-type semiconductor, by contrast, holes dominate conductivity.
Making p-type nitride semiconductor materials can be difficult generally, and obtaining conductive crystals or epitaxial layers of p-type aluminum nitride (AlN), or of AlxGa1-xNA alloys with high Al content, has posed particular challenges. Adding carbon and oxygen to AlN causes it to turn blue, which means it absorbs in the red (unlike more typical AlN, grown without added impurities, which tends to absorb in the blue due to N vacancies). Some conductivity measurements have suggested that the blue crystal is p-type while other work has cast doubt on the possibility of making p-type AlN at all. The acceptor levels from most substitutional dopants in AlN will tend to be rather deep in the energy bandgap, making it difficult to achieve reasonable conductivity levels unless high concentrations of the dopant are used. Unfortunately, the solubility of a single p-type impurity atom tends to be rather low, and tendency of the crystal to form charge compensating vacancy defects will be high.
In any case, the only p-type AlN materials produced to date have involved small crystals, only a few millimeters (mm) in size, grown in the laboratory. N-type doping of nitride materials also presents difficulties. Thus, success in creating large, conductive crystals has proven elusive.